How Does a Thermostat Work

How does a thermostat work? How can one little switch control the temperature throughout your entire house? Knowing how a thermostat works can also help you save energy (and therefore money).

A thermostat works using a mercury switch. That's the little glass part filled with liquid (like old-school thermometers). Mercury, as a metal, conducts electricity. Inside the mercury switch are three wires; one which rests in the mercury, therefore keeping electricity moving and two which are on either side of the switch and come into contact with the bottom wire when tilted. The wire on the left side of the mercury switch controls the heat while the wire on the right side of the mercury switch controls the air conditioning. A circuit board connects the wires from the mercury switch to your heater and air conditioner.

A thermometer coil rests against this mercury switch and determines which way the switch is going to tilt (and set off either the heat or air conditioning). The thermometer coil is made of copper and iron which each respond to different temperature levels, meaning they'll signal your thermostat to start working depending on the room temperature. When you adjust your thermostat, you're tightening the thermometer coil. The more you adjust the thermostat the more the coil tightens and the greater the change in temperature will be.

A digital thermostat works similarly to a standard mechanical thermostat but its electrical resistance changes with changes in temperature. A digital thermostat can be programmed, unlike a mechanical thermostat. They're also generally easier to read and therefore slightly more efficient at controlling the temperature in your home.